The invention concerns the electrical circuitry of magnetostrictive position sensors and in particular an electronic arrangement for the production of electrical current pulses in an ultrasonic exciter circuit of a magnetostrictive position sensor and for evaluation of the ultrasonic pulses on a magnetostrictive waveguide in order thereby to determine the relative position of a position marking (magnet) which is displaceable on the magnetostrictive waveguide.
Electrical pulses are required to operate the most widely varying range of electrical devices and apparatuses. Such a device is an ultrasonic position sensor in which an electrical current pulse is passed through an electrical conductor which is surrounded by a magnetostrictive waveguide such as for example a magnetostrictive tube. An external magnet serves as a position marking which is displaceable along the magnetostrictive waveguide and generates a stationary magnetic field which interacts with the circular magnetic field of the current pulse which is passed through the exciter circuit (electrical conductor). That interaction gives rise to an ultrasonic pulse in the waveguide which is propagated along the latter. That ultrasonic pulse is detected at a reference position, usually the end of the magnetostrictive waveguide, by a detector. The period of time between triggering of the exciter current pulse and reception of the ultrasonic pulse is a measurement in respect of the distance of the displaceable position magnet from the detector.
A distinction is to be made in respect of the following circuits, for electrical control of such an ultrasonic position sensor and for evaluation and processing of the ultrasonic waves which pass along the magnetostrictive waveguide:
a control circuit for generating an electrical control pulse with which a pulse generator is controlled,
a pulse generator for generating an electrical current pulse in the ultrasonic exciter circuit of the ultrasonic position sensor,
an ultrasonic pulse detector for generating an electrical reply signal from the ultrasonic pulse which was produced by the exciter pulse, and
an evaluation circuit for evaluation and further processing of the electrical reply signal, with which the relative position of a displaceable position magnet can be ascertained.
The above-mentioned operational circuits are generally connected to a supply potential. Furthermore it is known for regular control pulses to be transmitted from the control circuit to the pulse generator by way of separate lines and for the electrical reply signals to be transmitted to the evaluation circuit by a further conductor. Therefore up to six lines are required for the control and reply signals (useful signals) and the supply voltage, including the respective return conductors. The above-mentioned lines are generally combined in a cable which leaves the housing of the ultrasonic position sensor through an opening. The thickness of the cable means that the corresponding opening must be correspondingly large, which on the one hand makes it difficult to achieve sealing integrity in relation to harmful environmental influences while on the other hand representing a weak location in terms of compliance with EMC-requirements (electromagnetic compatibility).
a) Technical Object
Therefore the object of the present invention is to supply the individual operational circuits of the electronic system of a magnetostrictive sensor by way of a cable of thinner cross-section and to transmit both control and also reply signals (useful signals) by way of that cable.
b) Attainment of the Object
In accordance with the invention that object is attained by the features recited in the characterising portions of the independent claims. Advantageous configurations of the invention are set forth in the appendant claims.
The fact that the steady supply voltage is superimposed with frequent control and reply signals on the supply potential conductor means that a two-pole line is sufficient for transmission of the signals. If, by virtue of the arrangement of the individual circuits, the signals have to be transmitted bidirectionally by way of the two-pole line, it is possible to make use of the fact that there is a certain period of time, more specifically precisely the transit time of the ultrasonic wave on the magnetostrictive conductor, between the generation of a control pulse and transmission of the reply signal in the opposition direction, whereby the signals cannot interfere with each other. The clock frequency of the control pulses is to be selected to be sufficiently great.
The method according to the invention includes the following steps:
generating a current pulse in order to trigger a current pulse in the exciter circuit of a magnetostrictive position sensor,
transmitting the control pulse to a switch of a pulse generator,
triggering the current pulse in the exciter circuit of the magnetostrictive position sensor,
detecting and converting an ultrasonic pulse into an electrical reply signal, and
transmitting the reply signal to an evaluation circuit in order to determine the relative position of a magnet on the waveguide.
The circuits required are disposed in part inside and in part outside the housing of the position sensor. If the useful signals (control pulse and reply signal respectively) are transmitted into the housing or in the opposite direction respectively by virtue of the arrangement of the circuits, that is preferably implemented by way of a two-pole electrical line, with the useful signal in question being superimposed on the supply potential. At least the useful signals from and to the evaluation circuit are transmitted by way of the two-pole supply line.
Coupling-out and transmission of the useful signals on the supply conductor is not absolutely necessary between circuits which are disposed either jointly within or outside the position sensor housing. If a useful signal within one of the regions (inside or outside the housing) is to be transmitted to another circuit, that can also be implemented by way of a direct line.
The control pulse generated by the control circuit is preferably transmitted to the evaluation circuit on the supply conductor and used for initialisation of the evaluation circuit, that is to say for example a flip-flop is set and a counter is started, as is known for example from U.S. Pat. No. 4,721,902 to Tellerman et al. The signal received after a given transit time of the ultrasonic waves at the ultrasonic pulse detector is converted into an electrical reply signal and goes back to the evaluation circuit by way of the supply potential conductor, preferably a two-pole line such as for example a coaxial line, in the opposite direction to the control signal. It is however also propagated in the direction of the pulse generator on the supply line.
In order to prevent the reply signal once again triggering a current pulse in the ultrasonic exciter circuit, it is relatively weakly amplified in the ultrasonic pulse detector and is of a substantially lower amplitude than the control signal. That means that the switching threshold of a switch in the ultrasonic exciter circuit is not exceeded.
The individual circuits of the electronic system of an ultrasonic position sensor arexe2x80x94if necessaryxe2x80x94preferably respectively connected at their supply connections to the supply conductor and are thus at the supply potential. Inputs and outputs which serve for the transmission of useful signals by way of the supply conductor are decoupled from the supply potential by suitable impedances such as for example capacitance means. The supply connections of the individual circuits in contrast are decoupled from high-frequency signals by suitable filters such as for example inductors or LC-low pass filters in order to guarantee a stable current supply and in order to decouple the useful signals to be transmitted from the low-resistance paths of the current supply.
A coaxial line is preferably used as the supply line. The arrangement of the individual circuits with respect to the coaxial line is variable, in which case for example the control circuit can also be arranged to the right of the coaxial line in FIG. 1. Likewise the pulse generator, excluding the switches 7, can be arranged to the left of the coaxial line 5. In this case the circuits to the right of the line would be at the increased output voltage of a booster device which is part of the pulse generator. That voltage can be regulated to the desired value for example by voltage regulators for the circuits arranged on the right-hand side in FIG. 1.
A fundamentally different solution proposes a two-pole line for signal transmission, on which however there is no dc voltage as a supply voltage. The energy necessary for powering the individual circuits is ensured in particular solely by the energy of the exciter current pulses. A charge storage means, preferably a storage capacitor, is charged up with a part of the energy of the exciter current pulse. The voltage at the capacitor can be fed to other circuits such as for example the pulse detector as a supply voltage.